1. Field of the Invention
The present invention relates to an analog front-end device that is provided between a solid-state image pickup sensor and a digital signal processor in a digital camera. More specifically, the present invention relates to a technique for suppressing influences of digital noise when transferring image data.
2. Description of the Related Art
Recently, there has been a conspicuous shift from the analog technology to the digital technology in the camera industry. In particular, digital still cameras that require no film or development thereof have grown into a brisk business. Portable telephones with built-in cameras have become the main stream. The number of pixels has grown more and more and there has been a remarkable improvement in the picture quality obtained through image processing.
FIG. 5 is a block diagram for showing the structure of a conventional analog front-end device. Reference numeral 20 is a solid-state image pickup sensor that is a MOS-type image sensor, 30 is a digital signal processor (DSP) for performing image processing, A′ is an analog front-end device, 1 is a synchronous signal generator for generating periodic synchronous signals, 2 is a timing generator for generating a drive pulse for the solid-state image pickup sensor 20, 3 is a preprocessor, 4 is a CDS (correlated double sampling) circuit, 5 is a GCA (gain control amplifier), 6 is an A/D converter, 7 is a digital data output device, 8 is a CPU interface, and 9 is a clock multiplier which multiplies and outputs clocks inputted from the outside.
In the conventional analog front-end device A′, when imaging light enters the solid-state image pickup sensor 20 through a lens (not shown), it is converted into electric signals by a photodiode or the like, and image signals that are analog continuous signals are generated by vertical and horizontal drive pulses from the timing generator 2. The generated image signals have the 1/f reduced properly are processed in a sample hold circuit of the CDS circuit 4. Then, the image signals have the gain controlled in the GCA 5, and are converted into digital image signal data (RGB data) by the A/D converter 6. The converted image signal data is outputted in the parallel data form or outputted after being converted from the parallel form into the serial form, from the digital data output device 7 to the external digital signal processor 30, where various kinds of processing such as luminance signal processing, color separation, color matrix processing is executed.
When an electronic shutter mechanism of the solid-state image pickup sensor 20 does not employ a whole-pixel simultaneous readout system but employs a rolling shutter system of a successive readout type, an electronic shutter drive control pulse is outputted for each line.
If an exposure control period is less than one horizontal period when operating the electronic shutter at a high speed under a highlight environment, the control pulse generated by the timing generator 2 within a valid data period is inputted to the solid-state image pickup sensor 20, as shown in FIG. 6. The solid-state image pickup sensor 20 is driven by the control pulse, and CDS processing, GCA processing, and A/D processing is executed in the driven solid-state image pickup sensor 20 simultaneously with the valid data output period. The result of the processing is outputted from the digital data output device 7 to the outside as the output data. At that time, an operation noise is generated within the valid data period due to the output of the control pulse. The operation noise is generated during the valid period both in the solid-state image pickup sensor 20 and the analog front-end device A′.
In the conventional analog front-end device, output operations of the control pulses at arbitrary positions within the valid data period are executed as the simultaneous operations during the valid data output period, in a mode for operating the high-speed electronic shutter when the exposure control period is less than one horizontal period. Therefore, discontinuous noise components are generated due to the control pulse output operations, and the noise components give bad influences on each part (the inner side of the solid-state image pickup sensor, a connection part between the solid-state image pickup device and the analog front-end device, the timing generator, the CDS circuit, the GCA, the A/D converter, and the like of the analog front-end device). As a result, a fixed pattern noise (FPN) appears on the image (see “5”, “10”, “15”, “20” of the image data P0 in FIG. 6).